GRB 100418A is a long Gamma-Ray Burst at redshift z=0.6235 discovered with the Swift Gamma-Ray Burst Explorer with unusual optical and X-ray light curves ' After an initial short-lived, rapid decline in X-rays, the optical and X-ray light curves observed with Swift are approximately flat or rising slightly out to at least approx.7 ks after the trigger, peak at approx.50 ks, and then follow an approximately power-law decay. Such a long optical plateau and late peaking is rarely seen in 6R8 afterglows. Observations with REM during a gap in the Swift coverage indicate a bright optical flare at approx.25 ks, The long plateau phase of the afterglow is interpreted using either a model with continuous injection of energy into the forward shock of the burst or a model in which the 'et of the burst is viewed off-axis. In both models the isotropic kinetic energy in the late afterglow after the plateau phase is >100 times the 10(exp 51) erg of the prompt isotropic gamma-ray energy release. The energy injection model is favored because the off-axis 'et model would require the intrinsic $T f801$ for the GR8 'et viewed on-axis to be very short, approx.10 ms, and the intrinsic isotropic gamma-ray energy release and the true jet energy to be much higher than the typical values of known short GRBs^ The non-detection of a 'et break up to approx.2 Ms indicates a jet half-opening angle of at least 14 degrees, and a relatively high collimation-corrected 'et energy of at least 10(exp 52) erg.